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Group VII
__FORCETOC__ The Basics The facts that you learned at GCSE remain relevant: these were the colours ''and ''states ''at room temperature of the 'elements', their relative reactivity/ability to 'displace 'each other, and their colours when dissolved in water and in organic solvents like cyclohexane. You also learned that the difference in reactivity is due to differences in the ability of their 'atoms''' to attract the one extra electron needed to fill their outer shell. You will also have seen exam questions asking you to predict what Astatine 'would be like if only it could be made stable enough to study. It's not unlikely that you could be asked this again, but your answers would have to be a little more detailed. The tests for Halide ions (I-, Br-, Cl-) also have not changed, though there are a few additional details to learn. For details of the tests for Halide ions press 'here. For details of the reaction of Chlorine and water press here. Exam Hint thumb|right|300px|Examiners want you to say that Chlorine is Green (even though it often looks yellow at low concentrations). They want you to say Bromine is liquid and Iodine is a solid at Room Temp, even though both are quite volatile. State and explain the trend in Melting/Boiling points in Group VII marks #. The Melting/Boiling points increase down the group... # ... due to increased Van der Waals forces... #. ...which increase with molecular mass If you need to explain why the Van der Waals forces increase with molecular mass see Van der Waals forces. State and explain the trend in reactivity in Group VII marks At GCSE we explain that Fluorine is a small atom with few shells. It attracts electrons more strongly than other halogens because the electron can get closer to the positive nucleus. This is still a valid argument but you should now add that Fluorine's electrons are not highly shielded from the attraction of the nucleus. So marks might be awarded in an AS exam for; # Reactivity decreases down the group... #... as electrons are more strongly attracted to the nuclei of smaller atoms (where they can more closely approach the nucleus)... #. ...and which have fewer inner shells to shield the attraction of the nucleus. State and explain the trend in oxidising (or reducing) power in Group VII marks This is fundamentally the same question as the one above and so the same points would earn you 3 marks and you only need to add how the ability to gain electrons affects oxidising/reducing power. Fluorine is best at gaining electrons (and being reduced), so it is the most oxidising. Oxidising power increases up Group VII Iodine is best at losing electrons (and being oxidised), so it is the most reducing. Reducing power increases down Group VII. State your observations and write equations for the reactions (if any) when aqueous Chlorine is added to aqueous solutions of Sodium Fluoride, Sodium Bromide and Sodium Iodide to which cyclohexane has been added. Obviously, this question may involve different combinations of halogens and halide ions but it's important to note that there will always be at least one non-reaction and that there is a mark for stating this. NEVER FAIL TO STATE THE OBVIOUS. In the case of this particular question, marks might be awarded for: 1) There is no reaction between Chlorine and Fluoride ions. Both the water and the cyclohexane will remain colourless. 2) Chlorine will displace Bromine. The aqueous layer may be briefly orange/brown but the Bromine produced will accumulate in the cyclohexane, making it orange. 3) Cl2 + 2Br- => Br2 + 2Cl- 4) Chlorine will also displace Iodine. The aqueous layer may be briefly brown but the Iodine produced will accumulate in the cyclohexane, making it purple. 5) Cl2 + 2I- => I2 + 2Cl- Additional marks for similar questions are sometimes available for explaining that (in this case) the Chlorine is oxidising the Bromide and Iodide ions. And for explaining the halogens do not dissolve well inw ater because Halogen molecules are non-polar, but dissolve well in non-polar solvents like cyclohexane because it has similar Intermolecular forces - Van der Waals forcess in this instance. File:The Halogens External Links More Exam style questions http://www.s-cool.co.uk/a-level/chemistry/group-vii/test-it/exam-style-questions Multiple-choice questions http://www.s-cool.co.uk/node/2189/take Category:Periodicity